The present invention relates to a color picture display apparatus based on the Braun tube, and particularly to a white balance correction circuit for accomplishing the reproduction of the white color (white balance) on the screen.
In displaying monochrome pictures on the screen of a color picture display apparatus, it is required to reproduce achromatic colors of a wide luminance range on the screen. This is achieved through the adjustment of the electrode voltage or electron gun drive voltage of the picture tube, and it is called "white balance correction". Unless the white balance correction is implemented properly, color pictures cannot be displayed in correct colors.
Among a variety of white balance correction circuits that are already known, an example described in Japanese Patent Publication No. 57-51796 will be explained with reference to FIG. 5.
In the figure, the circuit receives a luminance signal Y on its input terminal 101, and the signal is conducted through a transistor 102 and resistors 105, 109 and 113 in emitter-follower configuration and delivered to the emitter electrodes of transistors 104, 108 and 112. The circuit also receives color difference signals R-Y, G-Y and B-Y on its input terminals 103, 107 and 111, and these signals are delivered to the base electrodes of the transistors 104, 108 and 112. These three transistors function to subtract the luminance signal component from the respective color difference signals, and produce primary color signals R, G and B on their collector electrodes in connection with load resistors 106, 110 and 114. This circuit is generally called "output circuit".
The primary color signal B produced on the collector electrode of the transistor 112 is conducted through a circuit that is a parallel connection of a resistor 119 and a serially-connected resistor 121 and zener diode 120, and applied to the cathode electrode 124 of an electron gun for blue (B) of the color picture tube 125 so that the electron beam of blue is modulated by the signal. Similarly, the primary color signal G produced on the collector electrode of the transistor 108 and conducted through a circuit consisting of resistors 116 and 118 and a zener diode 117 is applied to the cathode electrode 123 of an electron gun for green (G) so that the electron beam of green is modulated by the signal. Another primary color signal R produced on the collector electrode of the transistor 104 and conducted through a resistor 115 is applied to the cathode electrode 112 of an electron gun for red (R) so that the electron beam of red is modulated by the signal. Each electron gun has its beam current intensity determined from the difference between the voltage of the cathode electrode and the voltage of the first grid electrode (not shown).
Assuming that all electron guns have their first grid maintained at zero volt, if the collector voltage of the transistor 104 associated with the red electron gun falls, the cathode voltage also falls, causing the red electron beam to increase. The beam current flows through the resistor 115, producing a voltage drop across it with a polarity opposite to the collector voltage of the transistor 104, thereby suppressing the beam current. Namely, the resistor 115 functions to provide a negative current feedback for the electron gun having nonlinear characteristics called "gamma" so that the linearity of operation is improved. Increasing the resistance of the resistor 115 increases the amount negative feedback, and thus more reduces the gamma and enhances the linearity of electron gun.
The zener diodes 117 and 120 have a certain threshold voltage (zener voltage) of operation, and each of these zener diodes turns on when the voltage across it reaches the zener voltage, or otherwise it turns off. When the beam currents of the green and blue electron guns are smaller, the voltage drops across the resistors 116 and 119 are lower than the zener voltage, causing the zener diodes 117 and 120 to be off. For the resistors 106, 110, 114, 115, 116 and 119 having resistances of R106, R110, R114, R115, R116 and R119, respectively, the red, green and blue electron guns operate in an equal condition of gamma and the white balance is satisfied if the condition R106+R115=R110 +R116=R114+R119 is met.
If the beam currents of the green and blue electron guns increase, causing the voltage drops across the resistors 116 and 119 to rise beyond the zener voltage, the zener diodes 117 and 120 turn on, resulting in a parallel connection of the resistors 116 and 118 in the cathode circuit of the green electron gun and a parallel connection of the resistors 119 and 121 in the cathode circuit of the blue electron gun. The resistors 121 and 118 have their resistances R121 and R118 selected to satisfy the following conditions: EQU (R114+R119//R121)&lt;(R110+R116//R118)&lt;(R106+R115)
where symbol "//" denotes the resistance of two resistors connected in parallel. Consequently, the blue electron gun has a large gamma, the red electron gun has a small gamma, and the green electron gun has a medium gamma between those of the red and blue electron guns.
Based on the foregoing circuit parameter setting, it is possible to have a large beam current for blue, a medium beam current for green and a small beam current for red for luminance signal levels above a certain threshold value, and it is possible to reproduce a picture on the screen of color picture tube such that the color temperature rises progressively for luminance signal levels above a certain level of the luminance signal, i.e., the accomplishment of white balance.
Development is under way for the enhancement of brightness and resolution of color pictures reproduced by display apparatus based on the Braun tube, particularly projection-type color picture display apparatus in which pictures reproduced by small independent Braun tubes for red, green and blue are projected by being magnified on to a screen.
The enhancement of brightness necessitates an increased electron beam current of the Braun tube and the enhancement of resolution necessitates a reduced spot diameter of the electron beam. However, both of these schemes increase the application power density on the fluorescent screen of the picture tube, promoting adversely the saturation of luminance of the fluorescent substance. The saturation of luminance of the fluorescent substance signifies that the light emission of fluorescent substance does not increase in response to an increased beam current in the large current region. This phenomenon of luminance saturation is more significant for the red fluorescent substance and particularly pronounced for the blue fluorescent substance relative to the green fluorescent substance.
There is known the following relationship between the electron gun drive voltage Ed and the anode current (beam current) Ep: EQU Ip=K.sub.1.(Ed).sup..gamma.1 (where K.sub.1 is a constant) . . . (2)
The parameter .gamma..sub.1 is called the gamma characteristics of the electron gun.
There is another relationship between the luminance B (Br, Bg, Bb) of fluorescent substance (red, green, blue) and the electron gun anode current I (Ir, Ig, Ib) as follows. EQU Bm=K.sub.2.(Ip).sup..gamma.p (where K.sub.2 is a constant, and m and p both represent subscripts r, g and b for red, green and blue) . . . (3)
The parameter .gamma..sub.p is the gamma value of a fluorescent substance, e.g., the red fluorescent substance has .gamma..sub.r.
The above formulas (2) and (3) are reduced to the following. EQU Bm=K.sub.3.(Ed).sup..gamma. (where K.sub.3 =K.sub.1.K.sub.2, and .gamma.=.gamma..sub.1..gamma..sub.p) . . . (4)
The parameter .gamma.=.gamma..sub.1..gamma..sub.p represents the gamma characteristics of luminance for a drive voltage Ed.
In case the electron gun has a gamma value .gamma..sub.1 of about 2.5 for example, the green fluorescent substance has the best characteristics of .gamma..sub.g .congruent.1.0 among the luminance characteristics Br, Bg and Bb of red, green and blue, providing a substantially constant gamma characteristics of .gamma..sub.1..gamma..sub.g .congruent.2.5 throughout the luminance range. The red fluorescent substance has a constant gamma characteristics (.gamma..sub.1..gamma..sub.r .congruent.2.5) in a low luminance region, but it exhibits the saturation (.gamma..sub.1..gamma..sub.r .congruent.1.8) in a high luminance region. The blue fluorescent substance has .gamma..sub.1..gamma.b.congruent.1.6 in a low luminance region, but it exhibits the saturation (.gamma..sub.1..gamma.b.congruent.2.3) in a high luminance region. Due to the different gamma values .gamma.r, .gamma.g and .gamma.b of the fluorescent substances of red, green and blue, the white balance of the picture tube varies in response to the variation of luminance.
In the case of the ordinary color television receiver in which images of red, green and blue are formed on a single Braun tube, the light output is proportional to about the 2.2-th power of the signal voltage (luminance signal) applied to the grid of picture tube. Accordingly, it is possible to restore the linearity of this input/output relation through the modification of the signal voltage by means of a circuit that produces an output voltage in proportion to about the 0.45-th (1/2,2-th) power of the input signal voltage before it is applied to the picture tube. Actually, the NTSC-based color television signal has the rendition of the 0.45-th power of input voltage at the broadcasting station, and therefore each television receiver does not need to equip the above-mentioned modification circuit for the reproduction of satisfactory color pictures.
However, the projection-type display apparatus based on small independent Braun tubes for red, green and blue is required to produce a large light output, and the problem of the saturation of fluorescent substance, which is not a concern of the ordinary color television receiver, emerges. Namely, in dealing with the Problem of saturation of fluorescent substances of red and blue, a well-balanced color picture cannot be reproduced from the NTSC-based color television signal having the gamma modification of about 0.45-th power for all colors unless the red and blue signals are given gamma values large enough to compensate the saturation in contrast to the green signal that does not have the saturation problem.
Despite the above-mentioned technical situation attributable to the enhancement of the brightness of screen, the foregoing prior art can merely relax the gamma characteristics, but is incapable of attaining large gamma characteristics. In a conceivable case of the foregoing prior art applied to the drive circuits of red and green with the intention of setting the gamma characteristics to around 1.8, it is not possible to provide the inherent tone characteristics for the luminance signal and another problem of degraded picture quality will arise.